If you are getting this error either with update or activation (like I was) just check that your TIME and DATE are set correctly. This resolves the error.
Your error message might also say something like below:
The validation service is unable to determine whether your copy of Windows Vista is genuine. This issue may be resolved if you run validation later. Until your copy of Windows is validated, certain features of Windows Vista may be disabled. Access to the latest updates, all available features, and product support is reserved for customers running genuine Windows.